1. Field of the Invention
The present invention relates to an organic electroluminescent display device, and more particularly, to an active matrix electroluminescent display devices and a method of fabricating the same.
2. Discussion of the Related Art
Currently, the need for flat panel displays having thin profiles, lightweight, and lower power consumption has increased. Accordingly, various flat panel display (FPD) devices such as liquid crystal display (LCD) devices, plasma display panels (PDPs), field emission display (FED) devices, and electroluminescence display (ELD) devices are being developed now.
Among the many different types of FPD devices, the electro luminescence display (ELD) device is the only one that makes use of electroluminescence phenomenon in which light is generated when an electric field is applied to a fluorescent substance. The electroluminescence display (ELD) devices can be classified into inorganic electroluminescence display (ELD) devices and organic electroluminescent display (ELD) devices depending on what type of source excites carriers in each of the devices. The organic electroluminescent display (ELD) device can display colors within a range of visible wavelengths and has a high brightness and a low action voltage. In addition, since the organic electroluminescence display (ELD) devices are self-luminescent, they have a high contrast ratio and are suitable for ultra-thin type display devices. Moreover, since they have a simple manufacturing process, environmental contamination during manufacturing is relatively low. Furthermore, the organic electro luminescence display (ELD) devices have a few microseconds (μs) response time so that they are suitable for displaying moving images. The organic electroluminescence display (ELD) devices are not limited by their viewing angle, and are stable under low temperature operating conditions. Accordingly, they can be driven with a relatively low voltage (between 5V and 15V), thereby simplifying manufacturing and design of corresponding driving circuitry.
Structures of the organic electroluminescent display (ELD) devices are similar to the structures of the inorganic electroluminescence display (ELD) devices, but the light-emitting system is different from that of the inorganic electroluminescence display (ELD) devices. For example, the organic electro luminescent display (ELD) devices emit light by a recombination of an electron and a hole, whereby they are often referred to as organic light emitting diodes (OLEDs). In addition, active matrix type systems having a plurality of pixels arranged in a matrix form with a thin film transistor connected thereto has been widely applied to the flat panel display devices. The active matrix type systems are also applied to the organic electroluminescent display (ELD) devices and are commonly referred to as an active matrix organic electroluminescent display (ELD) device.
FIG. 1 is a cross sectional view of an active matrix organic electro luminescent display device according to the related art. In FIG. 1, a buffer layer 11 is formed on a substrate 10, and a first polycrystalline silicon layer having first to third portions 12a, 12b, and 12c and a second polycrystalline silicon layer 13a are formed on the buffer layer 11. The first polycrystalline silicon layer is divided into the first portion 12a (i.e., an active region) where impurities are not doped, into the second portion 12b (i.e., a drain region), and into the third portion 12c (i.e., a source region) where the impurities are doped. The second polycrystalline silicon layer 13a functions as a capacitor electrode.
A gate insulation layer 14 is disposed on the active region 12a, and a gate electrode 15 is disposed on the gate insulation layer 14. A first interlayer insulator 16 is formed on the gate insulation layer 14 and the gate electrode 15, while covering the drain and source regions 12b and 12c and the second polycrystalline silicon layer 13a. A power line 17 is disposed on the first interlayer insulator 16 above the second polycrystalline silicon layer 13a. Although not shown, the power line 17 extends along one direction as a line. The power line 17, the second polycrystalline silicon layer 13a, and the first interlayer insulator 16 form a storage capacitor. A second interlayer insulator 18 is formed on the first interlayer insulator 16 to cover the power line 17.
First and second contact holes 18a and 18b penetrate both the first and second interlayer insulators 16 and 18 to expose portions of the drain region 12b and source region 12c, respectively. In addition, a third contact hole 18c that penetrates the second interlayer insulator 18 is formed to expose a portion of the power line 17. A drain electrode 19a and a source electrode 19b are formed on the second interlayer insulator 18, whereby the drain electrode 19a contacts the drain region 12b through the first contact hole 18a, and the source electrode 19b contacts both the source region 12c and the power line 17 through the second contact hole 18b and through the third contact hole 18c, respectively.
A passivation layer 20 is formed on the drain and source electrodes 19a and 19b and on the exposed portions of the second interlayer insulator 18. The passivation layer 20 has a fourth contact hole 20a that exposes a portion of the drain electrode 19a. A first electrode 21 that is made of a transparent conductive material is disposed on the passivation layer 20 to electrically contact the drain electrode 19a through the fourth contact hole 20a. A bank layer 22 is formed on the first electrode 21 and on the exposed portions of the passivation layer 20, and has an opening 22a (often referred to as a bank) that exposes a portion of the first electrode 21. An electroluminescent layer 23 is formed in the bank 22a of the bank layer 22. On the exposed portions of the bank layer 22 and on the electroluminescent layer 23, a second electrode 24 is formed of an opaque metallic conductive material.
In FIG. 1, the first electrode 21 is formed of the transparent conductive material, and the second electrode 24 is formed of the opaque conductive material. Accordingly, the light emitted from the organic electroluminescent layer 23 is released along a bottom direction, which is commonly called a bottom emission-type device.
FIG. 2 is an enlarged cross sectional view of a portion A of FIG. 1 according to the related art, and FIG. 3 is a plan view of the enlarged portion A of FIGS. 1 and 2 according to the related art. In FIGS. 2 and 3, the organic electroluminescent layer 23 is generally formed of a high molecular substance, whereby a solvent dissolves the high molecular substance and the dissolved high molecular substance is deposited into the bank opening 22a and on the bank layer 22 by an ink-jet technique. Then, the liquid high molecular substance on the bank layer 22 flows into the bank opening 22a during a heat treatment process. As a result of the heat treatment process, the organic electroluminescent layer 23 is formed in the bank opening 22a, and the solvent and other impurities contained in the liquid high molecular substance are removed. However, since the bank layer 22 is an organic material, such as one of the polyimide groups that have good interface characteristics with the high molecular substance, the electroluminescent layer 23 of the high molecular substance is positioned not only on the first electrode 21 but also on the bank layer 22 around the bank opening 22a, especially on side and top surfaces of the bank layer 22.
To prevent the electroluminescent layer 23 from being formed on the surfaces of the bank layer 22, a plasma treatment may be conducted on the electroluminescent layer 23 of high molecular substance. However, the plasma treatment causes the electroluminescent layer 23 to have poor interface characteristics with the first electrode 21, thereby preventing adequate bonding to the first electrode 21, Accordingly, the electroluminescent layer 23 delaminates due to thermal stresses when the electroluminescent layer 23 is operated for a long period of time, thereby significantly reducing its operational life span.
In addition, since the electroluminescent layer 23 is disposed on inclined side and top surfaces of the bank layer 22, as denoted by portions C in FIGS. 2 and 3, the light emitted from the portions C of the electroluminescent layer 23 has abnormally paths, as compared to the light emitted from a portion B where the electroluminescent layer 23 is disposed on the first electrode 21. The light generated in the portions C provides different spectral distributions of red, green, and blue light, whereby the red, green, and blue colors are refracted and disperse. Accordingly, it is difficult for the organic electroluminescent display devices shown in FIGS. 1, 2, and 3 to achieve white balance and to obtain a gray level display.